This invention relates generally to the field of heat treatment of metals, and more specifically to a method and system of providing real-time, closed-loop control of an induction heating machine by using a miniature magnetic sensor to measure the local changes in magnetic field close to a workpiece during induction heating.
Induction heating is a well-known process for efficiently applying energy directly to metals and other conductive materials for heat treating, melting, welding, brazing, tempering, normalizing, aging, or pre-heating prior to hot working.
Induction heating can also be used in non-metal applications, including adhesive bonding, graphitizing carbon, drying, curing, and superheating glass. In induction heating, alternating electric current is passed through an induction heating coil that is positioned closely to a workpiece. Where the lines of magnetic flux produced by the induction heating coil enter the workpiece, the alternating magnetic fields induce an alternating electric potential (e.g. voltage) in the workpiece. The alternating electric potential drives eddy currents in a thin surface layer. These eddy currents dissipate some of their energy within the surface layer by resistive Joule heating losses. The depth of resistive heating (e.g. skin depth) is inversely proportional to the square root of the product of three parameters: applied induction frequency, magnetic permeability, and electrical conductivity. The resultant temperature rise in the resistively heated surface layer is related to the specific heat, density, thermal conductivity, power level, and duration of heating. Magnetic coupling of the induction heating coil to the workpiece depends strongly on the geometrical arrangement, among other properties.
A common use of induction heating is case hardening of medium-carbon steel parts, such as gears, axles, and driveshafts. Many industrial applications require a steel part having a hardened outer surface (e.g. xe2x80x9ccasexe2x80x9d) and an interior region of higher toughness to provide improved strength, wear resistance, fatigue life, and toughness. Other applications include induction hardening of crankshafts, valve seats, railroad rails, rolling-mill rolls, and hand tools. Induction heating rapidly heats the outer surface layer of the steel workpiece in a short period of time (e.g. 5 seconds). Above a critical transition temperature (about 760 C for 1050M steel with 0.45% C) the initial ferrite-pearlite microstructure (BCC) transforms into the austenite phase (FCC).
Upon continued heating of the part, the transformed austenite layer thickens and extends deeper from the surface. Optimum peak surface temperatures can be 870-925 C, depending on the carbon concentration, and the desired depth of hardening. For some applications, the peak surface temperature can be as high as 1200xc2x0 C. Final hardening of the outer layer occurs when the heating power is shut off and the part is quenched (e.g. rapidly cooled from the outside to less than 200-400xc2x0 C. in 10-20 seconds). This converts the austenitic layer into a hard, metastable martensitic phase with a Rockwell hardness of Rc=50-60. An optional tempering step can follow the quench cycle, which can further improve the metallurgical properties.
Induction hardened steel parts are designed to have a case hardened layer with a specific desired depth. For example, a 25 mm diameter 1050 M steel automobile axle may be designed with a hardened layer from 4-5 mm thick, as defined by a Rockwell hardness of at least Rc=50. Should the layer be too thin, the axle would wear too quickly or have insufficient strength; should the layer be too thick, the axle would be too brittle. During mass production, the measured case depth should be repeatable to within +/xe2x88x920.1 mm. This requires close control of the induction heating process, as well as tight control of material properties, chemistry, workpiece alignment, etc.
Closed-loop control of the induction heating and hardening processes has been an elusive goal of the industry for many years. Existing induction hardening equipment is typically operated with open-loop process controllers, wherein an operator manually selects power and time (e.g. heating duration). Production users of this equipment monitor the process by destructively sectioning finished parts and inspecting the results; i.e., a finished part is cut apart and the case depth is directly measured radially across the cross-section by using a Rockwell hardness indentor, metallographic inspection, or chemical analysis of the carbon concentration profile. Process development for new parts is accomplished by time-consuming and expensive trial-and-error; for a given coil and part design, heating and quenching parameters are varied until destructive analysis reveals that the desired hardness profile is being produced.
These parameters are then utilized in the production run and the hardened parts are sampled and analyzed at regular intervals for quality control and assurance. If the tested part is bad, the production run from the previously tested good part is sampled to determine where the process failed. Production equipment may be taken out of service until subsequent parts test satisfactory. Since each test can take a minimum of several minutes by a trained technician, this process is quite inefficient for mass production. Unfortunately, small variations in the steel""s chemistry and microstructure can produce unacceptably large variations in the measured case depths, even for nominally acceptable material specifications. The cause of these variations is not well understood.
Other sources of variability include improper part positioning (e.g. misalignment relative to the heating coil), defects in the part (e.g. cracks), and damaged or aged heating coils. Low hardness values measured on a finished part may be caused by: surface decarburization; lower carbon content than specified; inadequate austenitizing temperature; prior structure; retained austenite (mostly in high-carbon steels); and unsatisfactory quenching.
Eddy current testing is a commonly used non-destructive method in the automotive manufacturing industry for measuring case depth. Eddy current testing can measure case depths in hardened shafts over a range from 0.2 mm to 9 mm, with an accuracy of about 0.15 mm RMS error. See Automotive Application of Eddy Current Testing, in Electromagnetic Testing, Vol. 4, 2nd ed., Nondestructive Testing Handbook, American Society of Nondestructive Testing, Inc., 1986, p 424-426. However, to achieve this level of accuracy requires the use of a master shaft for calibration purposes. This requires destructive measurement of the case depth in the master shaft by conventional hardness scans. All eddy current responses for the batch of test shafts are then normalized to the response for the master. A computer uses the responses at a few different frequencies to estimate the case depth using multiple linear regression fits. However, this type of eddy-current test is only performed after the part has been induction hardened (e.g. on finished parts); it is not used to provide real-time process control.
What is needed is a real-time, non-destructive, non-contact diagnostic technique that can respond quickly to the temperature changes and phase transformations in the workpiece during the induction heating process. The diagnostic should be small enough to provide sufficient spatial resolution, and robust enough to withstand the hostile environment (high temperatures, high fields, large volumes of quenching fluids, etc.). Use of an active feedback of process information measured directly from the part, coupled with closed-loop control of the heating process, would greatly improve the efficiency of induction hardening systems, while increasing accuracy and reducing part rework.
Direct measurement of the workpiece""s surface temperature during induction heating could provide a useful signal for closed-loop feedback control. However, use of contact thermocouples is impractical for mass production, especially since cylindrical parts are often rotated at significant rpm""s to create uniform heating profiles. Non-contact optical pyrometry could be used, however the accuracy is affected by surface conditions (e.g. emissivity) and the operating environment (e.g. smoke, dust, vapors). Coating of the pyrometer""s window by the quenching fluid can also degrade accuracy. Commercially available pyrometers do not have a sufficiently fast response time to monitor the rapid changes in surface temperature during induction heating. Neither pyrometry, nor surface-attached thermocouples, can directly measure the internal temperatures within a workpiece.
Indirect measurement of the workpiece""s temperature, and/or temperature profile through the depth, can be inferred by measuring corresponding changes in the electrical and magnetic properties of the workpiece as it heats up during induction heating. It is well known that the electrical resistivity increases with temperature for typical metals, including steel. For example, the resistivity of medium-carbon steels can increase as much as 800% as the temperature increases from 20xc2x0 C. to 900xc2x0 C. See ASM Handbook, Vol. 4, Heat Treating, 1991, p. 187.
The average electrical resistance of the workpiece (e.g. averaged over the cross-sectional area) can be measured indirectly by monitoring the voltage, current, and phase of the induction heating coil. This approach is described in U.S. Pat. No. 5,630,957 (commonly assigned to Sandia Corporation), which is herein incorporated by reference. In this patent, Adkins et al. teach a method of closed-loop control of an induction hardening machine that uses a trained neural network processor, combined with real-time measurement of the voltage, current, and phase in the induction coil, as measured by a Rogowski coil surrounding a current lead. The depth of hardening is controlled, in part, by computing the energy absorbed by the workpiece, and the changes in the average resistance of the coil plus the workpiece during the heating duration. However, this method does not provide any information regarding the temperature profile through the depth, or local information at a specific point on the workpiece.
A non-contacting, miniature magnetic sensor could be used for measuring the changes in surface magnetic fields near the workpiece in real-time during induction heating. A magnetic sensor responds to a time-varying magnetic field by generating a time-varying EMF (e.g. voltage) in the sensor""s monitor coil. As the workpiece heats up, changes in the electrical, magnetic, and microstructural properties of the heated surface layer affect the surrounding surface magnetic fields. A magnetic sensor positioned in close proximity to the surface could detect these changes. The output signals from such a sensor could provide useful information for controlling and optimizing the operation of an induction heating machine.
Magnetic sensors can be divided into two groups: active and passive. Active sensors provide their own excitation fields by using a driving coil (e.g. transmission coil or excitation coil). A second sensor coil (e.g. a monitor coil or probe coil responds to the time-varying magnetic field generated by the driving coil, which it is coupled through the workpiece. The excitation frequency of active sensors could be independently varied (e.g. 0-10 MHz), and could be much faster than the frequency of induction heating (e.g. 7 kHz). Use of a variable excitation frequency could provide the ability to probe the workpiece at varying depths, since the skin depth is inversely proportional to the square root of the driving frequency. Active sensors could also be used when the induction heating coil is temporarily at rest (e.g. during periodic pulsed heating), or during the cooling cycle (e.g. during quenching), when the induction heating coil is turned off. This is because active sensors provide their own source of excitation.
Active magnetic sensors can utilize a ferrite core to concentrate the magnetic flux, which improves overall performance. In this case, the monitor coil can sense four components of the total magnetic field: (1) the induction heating field, coupled through the workpiece and the ferrite core; (2) the magnetic field produced by eddy currents in the workpiece, in response to the induction heating field; (3) the high frequency excitation field, coupled through the ferrite core and workpiece; and (4) the magnetic field produced by eddy currents induced in the workpiece by the high frequency excitation field. Generally, the very small currents induced in the ferrite core can be neglected because of its high electrical resistance. Therefore, the only significant eddy currents are those inside of the workpiece. The magnetic loop includes the ferrite core and some portion of the workpiece. The magnetic field in the ferrite core depends, therefore, on all of the fields generated inside the workpiece, coupled through the magnetic loop.
Frequency filters could be used to eliminate either the high frequency (e.g. sensor excitation) or the low frequency (e.g. induction heating) components, as well as to control electromagnetic interference (EMI). Also, examination of the phase shift could be used to distinguish between these different magnetic components. Additionally, changes in the orientation of the excitation coil, the monitor coil, the ferrite core, and/or the workpiece could be used to selective emphasize either the coupled applied field, the eddy-current field, or both.
Passive magnetic sensors, on the other hand, do not have an independent excitation coil. Rather, they respond xe2x80x9cpassivelyxe2x80x9d to time-varying changes in the local surface magnetic field produced by two sources: (1) the magnetic field of the induction heating coil interacting (e.g. coupling) with the workpiece, and (2) the magnetic fields generated by the induced eddy currents that heat the workpiece. Consequently, the frequency measured by the passive magnetic sensor is nominally fixed by the induction heating frequency (e.g. 7 kHz). Despite the fixed frequency limitation, a passive sensor could be simpler, less expensive, and easier to instrument than an active sensor. Passive sensors could also detect the Curie temperature effect (to be discussed later).
Passive magnetic sensors could be used to monitor intra-cycle changes (e.g. during an active heating cycle) in the surface magnetic field during induction heating.
Magnetic sensors could be miniaturized (e.g. 1-2 mm diameter coil), to provide enhanced spatial and temporal resolution. Additionally, multiple sensors could be placed at various axial locations along an axle or driveshaft to monitor axial variations in the process. This could be applied for a continuous hardening process, where the workpiece is moving sequentially through a fixed set of induction heating coils and quench stations. Alternatively, multiple sensors could be used avantageously for complicated parts that are being heated simultaneously with multiple induction heating coils, each being controlled by individual controllers coupled to their own sensors.
Although the use of active magnetic sensors have been proposed for controlling induction heating machines, numerous problems exist with these methods. In U.S. Pat. Nos. 5,250,776 and 5,373,143, Pfaffmann teaches a method of using an eddy current sensor to xe2x80x9cmeasurexe2x80x9d the temperature of a part during the induction heating process. The method relies on the known increase in electrical resistivity as the workpiece heats up, causing a corresponding change in the impedance of an electromagnetic test coil placed adjacent to the metal part. See Introduction to Electromagnetic Nondestructive Test Methods, by H. L. Libby, John Wiley and Sons, Inc., 1971, p. 272.
Pfaffmann teaches that because of significant electromagnetic interference (EMI) produced by the induction heating machine, useful analysis of the eddy currents sensed by the eddy current sensor is impaired and, hence, real-time monitoring is not attainable. To get around this problem, Pfaffmann""s method specifically restricts the use of the eddy current sensor to periods of time when the induction heating coil is deliberately turned off.
Pfaffmann""s method is illustrated in FIG. 1. Here, the induction heating coil power is turned off at periodic intervals for short periods of time (e.g. 10 milliseconds). During the period of no heating, the excitation coil of the eddy current sensor is energized, thereby inducing eddy currents in the workpiece, which are detected by the sensor coil. Pfaffmann thereby eliminates the problem of electromagnetic interference by operating the eddy current sensor only when the induction heating coil is deliberately turned off.
Unfortunately, Pfaffmann""s method eliminates the possibility of using a simpler and cheaper passive magnetic sensor, since there is no excitation field to drive the passive sensor when the induction heating coil is deliberately turned off. Another disadvantage of Pfaffmann""s method is that additional electronic equipment is required to create, control, and synchronize the timing of the coordinated patterns for turning on and off the induction heating power, while simultaneously activating the eddy current sensor, thereby adding additional costs and system complexity.
Important process information may not be gathered because the large magnetic field created by the induction heating coil is missing. For example, the saturation of the induced magnetic field, Bsat (in ferromagnetic materials) inside of the heated surface layer is artificially missing when the induction heating coil is deliberately turned off. Additionally, commonly used commercial induction heating machines that operate on a continuous xe2x80x9charmonicxe2x80x9d cycle do not have a natural downtime in the heating cycle. Therefore, costly modifications of their electronics and control circuitry would be required to create the downtime period. Pfaffmann does not discuss the important effects of the Curie Temperature on the magnetic permeability, which strongly influences the induction heating process (to be discussed later).
In addition to detecting electrical properties, miniature magnetic sensors could be used to detect changes in the magnetic properties of a workpiece, including: (1) hysteresis in the magnetic permeability, and (2) the Curie temperature effect.
Changes in the relative magnetic permeability, mu, (e.g. relative to the free space permeability) that occur during heating are important to understand because the depth of induction heating is inversely proportional to the square root of the magnetic permeability. For soft ferromagnetic materials the permeability is a strong non-linear function of the applied magnetic field. The permeability is defined as the ratio of the Induced Field, B (Teslas), divided by the Applied Field, H (A/m). Above a certain applied field, the induced field saturates at an essentially constant value, Bsat, which is about 1.5-2 Teslas for medium-carbon steels. This is important because the magnetic field applied by the induction heating coil typically drives the surface of the workpiece well beyond magnetic saturation twice during each cycle (both positively and negatively). During saturation, when all of the magnetic domains align with the magnetic field, the induced eddy currents penetrate more deeply into the part because the permeability, mu, is much smaller inside the saturated zone.
Because alternating current drives the induction heating coil, the workpiece is subjected to an alternating applied magnetic field. Soft ferromagnetic materials respond with a hysteresis in their induced field, B, when the applied field, H, is cycled between maximum and minimum values, as shown in FIG. 2. Energy lost during AC magnetization is converted into heat in the ferromagnetic material, and can be represented, in part, by the area inside the hysteresis loop. The slope (e.g. permeability) of the hysteresis loop, and the flat-top (saturation field, Bsat), both depend on the temperature, driving frequency, carbon content, microstructure, and other properties of the workpiece. FIG. 3 shows an example of how a typical B-H hysteresis loop changes with temperature for 1050M medium-carbon steel at 5000 Hz. Increasing the temperature from 100 C to 700 C decreases the saturation field (Bsat) by roughly a factor of two. The permeability is also affected by temperature changes. These changes in magnetic properties significantly affect the heating profile through the depth, and, therefore, the temperature rise during induction heating.
Ferromagnetic materials undergo a dramatic transition from being a xe2x80x9cmagneticxe2x80x9d material with a large relative magnetic permeability (muxcx9c100-1000), to being a xe2x80x9cnon-magneticxe2x80x9d material (mu=1) when heated above the Curie temperature. The cause of the Curie effect is closely related to the phase transformation that occurs during heating, passing from a 100% ferrite-pearlite BCC microstructure when the temperature is below the Ac1line, to a 100% austenitic FCC microstructure when above the Ac3 line. For steels with carbon concentrations less than about 0.45 wt %, the Curie temperature is relatively constant at about 770xc2x0 C. In higher carbon steels the Curie temperature follows the A3 line on the Fe-C phase diagram to the eutectoid composition; thereafter, it coincides with the A1 line.
Both the Curie temperature and the ferrite-to-austenite phase transformation during heating are affected by many factors, including the rate of heating, the starting microstructure (e.g. annealed, normalized, quenched and tempered), grain size, carbon content, trace elemental composition, and possibly magnetic field, frequency, and stress state. When the heated surface exceeds the Curie temperature, austenite begins to form and the magnetic permeability rapidly drops to mu=1. Consequently, the induction heating magnetic field rapidly penetrates more deeply into the part. Continued heating increases the thickness of the austenite layer, until the desired depth is reached. Finally, the heating coil is turned off and the part is quenched, thereby forming a hard martensite surface layer.
Because the Curie temperature effect coincides closely with the beginning of austenite formation upon heating, the time after start of heating at which the Curie temperature is reached at the surface could be used as a sensitive indicator of how fast the workpiece is heating up. For example, if the Curie temperature is reached too quickly, this could indicate that too much power is being delivered to the workpiece, resulting in too great a case hardening depth. Likewise, if the Curie temperature signal is detected too late in the process, the case depth could be too shallow. Either condition could result in rejection of the part.
The heating process could be adjusted after the Curie temperature has been reached (and detected) by changing the power level of the induction heating coil, or by adjusting the heating duration (e.g. stop time), so that the desired depth of case hardening is precisely achieved. Because miniature magnetic sensors can detect the Curie temperature effect, they are well suited to provide critical information useful for actively controlling the induction heating process. This applies not only for induction hardening, but also for high temperature annealing or normalizing of steel and cast iron parts, using similar induction heating methods.
Additionally, analysis of the sensor""s response throughout the induction heating process could provide important information regarding the temperature profile through the depth, and the rate of heating over time, before the Curie temperature point has been reached. Likewise, similar information could be obtained during cooling when the Curie point is traversed when cooling down from a temperature above the Curie temperature point.